HMS HOOD: British 1919 Tests on Upper Belt and Deck Armor by Nathan Okun Introduction When HMS HOOD fought with and was destroyed by KM BISMARCK in its famous one and only battle at sea in 1941, the British had some idea of the effectiveness of the armor on its battlecruiser from calculations and several actual tests done just prior to freezing the final design of this ship. These tests were (1) against mock-ups of the multiple deck plating arrangement of HOOD above the forward magazine, both actual and proposed, and (2) against mock-ups, both actual and proposed, of the 7" Cemented Armor (CA, the latest, at the time, British form of the face-hardened side protection steel armor used in virtually all battleships and some large cruisers from the mid-1890s to about 1950) lower portion (strake) of the inclined upper belt, here bolted to a 1" High Tensile steel (HT, a modified mild/medium construction steel used by British ship-makers with an added small amount of nickel to allow higher hardness and strength without the steel cracking under stress during bad weather) ship outer hull construction bulkhead with 1" of cement in-between to act as a cushion and to seal out water. The latter also included all of the decks and the single thin vertical bulkhead plate between the rear surface of that CA plate and the upper spaces of the gun propellant powder magazine. The aft magazine was the same except for the lack of the uppermost weather deck, the forecastle deck that extended from the tip of the bow, and the lack of the associated upper 5" CA strake of the upper belt armor, which ended in a wedge shape just in front of the raised aft battery 15" "X" turret to allow the lower aft battery "Y" turret to fire somewhat forward of amidships, making the next deck under the forecastle deck , the upper deck riding on the upper edge of the 7" CA side armor, into the weather deck from there to the tip of the stern. This upper belt and the 12" inclined CA main waterline belt ended a few feet forward of the most-forward main armament turret, "A", and ended, minus its 5" top portion, as mentioned, a few feet aft of the aft-most turret, "Y"; this was the armored "Citadel" of HOOD. The original thicknesses of the various HT steel decks and internal unarmored bulkheads that were extended into the bow and stern regions never changed much, for the most part, from what I can see. Outboard of the waterline 12" CA main belt, covering all but the uppermost couple of feet of that belt and extending vertically down to the bottom of the hull was a relatively lightly-constructed outer hull with, just outboard of the belt-supporting strength bulkhead, a block of densely-packed rows of steel tubes below the waterline, designed to soak up the blast, shock, and water-hammer effect of a torpedo hit as they were crushed and torn -- the entire outer region was called the anti-torpedo "bulge" due to some older ships having this added after they were completed, so that a new, wider, outer side hull had to be layered onto the existing ship structure below the waterline. Thus, HOOD was one of the first ships to have in its final pre-construction design a recessed waterline belt spaced behind the visible hull made up of this bulge. (In WWII, many new battleships from several nations had this inclined-and- recessed belt concept in their side belt designs, with variations.) Below the lower edge of the inclined 12" main waterline belt amidships, the outer hull continued vertically down until it rounded inboard to form the bottom hull, this rounded portion being about two deck-heights from the flat bottom and extending about the same distance inboard from the side, making it a portion of a circular arc. Inboard of this, with a widening gap between it and the outer hull, was a continuation of the inclined HT steel bulkhead that supported the main belt to the bottom, meeting the bottom in that curved region just outboard of the flat bottom. A few feet inboard of that, also inclined the same way, was a second HT steel bulkhead, and spaced a few feet inboard of that, with a large gap at the top where it ended at the level of the bottom edge of the main belt, was the side vertical bulkhead of the powder and shell magazines. There were obviously other bulkheads and supports in all of those spaces, but these were the main structural vertical/inclined longitudinal plating below the waterline. Inside the magazine space was another thin HT steel deck, cutting it into two levels, one for powder and one for shells, with two more narrowly-spaced HT-steel 16 June 2019 Page 1 of 34 HMS HOOD: British 1919 Tests on Upper Belt and Deck Armor by Nathan Okun decks underneath forming a three-spaced-plate bottom hull for high structural strength. Inside the spaces of this bottom hull box were stored the ship's fuel oil, crew drinking water, and the distilled water needed by the ship's boilers. It also acted as ballast to keep the ship stable in bad weather or if it had battle damage and flooding, so as the fuel was used up, sea water was used to replace it. This liquid storage, with empty spaces used as air gaps, was also used in the outer side hull spaces adjacent to the magazine and amidships propulsion plant, forming the ship's anti-torpedo side protection system amidships. Note that the deck arrangement above and inside the propulsion plant, wedged between the two magazine spaces at each end of the ship and forming the major portion of the protected vitals, was different due to the large size of the boilers and engines requiring them to extend upward into the spaces above the waterline somewhat, which of course made them more vulnerable to enemy hits that punched holes in the side armor or protective decks. Since the ship had four engines and a large number of boilers, it could lose some and still function, if not at maximum speed, so such lower protection was accepted. HOOD thus had no portion of the amidships Citadel (powerplant and magazines) that was not on its side surface from about 5' below the standard waterline to the upper edge of the weather deck (either the long forecastle or the aft upper deck) protected by rather thick CA armor. (Underwater hits were considered unlikely at the time and thus not protected against if they occurred very far down the side hull, but this was true for virtually all warships when HOOD was built and even many later designs, which knew better, did not address this well, so this is not a major design flaw compared to many other major warships.) On top of that, the side hull form was such that the CA plating was sloped top-over- bottom by 12 degrees for the 12" recessed main belt, 20 degrees for the 5" uppermost strake of the upper belt, and by a curved 7" lower strake of the upper belt that was 12 degrees at its lower edge where it met the top edge of the 12" main belt and 20 degrees where it met the lower edge of the 5" upper strake (this was due to the lower hull narrowing near the ends of the ship but the weather deck kept as wide as possible as the ship narrowed to keep waves from flooding it). The entire 5" and 7" upper belt was at the same 12-degree tilt as the 12" waterline belt along the amidships propulsion plant length where the outer hull was closer to vertical. This high sloping of the side protection, even where it was not mandated by the waterline hull, such as near the bow, was also a new concept, designed to make the side armor significantly more resistant by requiring incoming enemy projectiles to hit the armor at a significantly increased oblique angle, which increased the resistance of the armor considerably at longer ranges, largely due to the hard face being much more able to break a penetrating projectile's middle and lower body as it slammed its side into the armor, even if the armor-piercing (AP) cap on the projectile nose worked to protect that portion of the shell. Face-hardened armor, while not as resistance to a hole being punched entirely through the armor plate as the softer homogeneous armors used for deck protection in later battleships to a well-designed AP shell when the AP cap worked properly, could much more severely damage a projectile as it tried to punch its way through the hole that its nose had made in the hard face, so forcing oblique hits can save the ship where a more right-angles hit would not, even if the velocity to punch through the plate was the same in both cases. Hardened AP caps were originally developed by the French circa 1911, I believe. They improved penetration of weaker-bodied AP projectiles over soft-capped designs and, when improved shells were also adopted, were found to work at any impact angle where they hit the armor first, causing these hardened AP caps to be continuously enlarged and thickened until they covered almost the entire shell nose by WWII and were, in some cases for WWII US Navy cruiser AP shells, as thick as the projectile diameter.